This invention relates to manufacturing electronic modules, and, more particularly, to a method and apparatus for efficient re-flow solder bonding of multiple populated circuit boards of a variety of different sizes and thickness within respective compartments in the module housing carrier, without use of custom-made clamping blocks.
Electronic equipment is often modularized, particularly that in the field of RF communication and at microwave frequencies and higher. That is, various electronic circuits are packaged together in a module and the module fits within (and is inserted into) a rack or other cabinet, alongside other modules, which collectively form an electronic system. Each module contains electrical connectors that plug into mating connectors in the rack or cabinet, enabling the various modules to communicate with other circuits in other modules via transmission lines in the respective rack or cabinet, and/or receive electrical power for operation. Typically, the module is constructed of a strong shallow rectangular metal frame, somewhat resembling a picture frame in appearance, and containing top and bottom metal walls, referred to as the cover and the carrier, that fasten to the frame and cover the rectangular window-like openings on top and bottom sides. Collectively, the foregoing frame and other components are often referred to as the module housing.
The metal walls and frame of the module housing shield the electronic circuitry inside the module from outside RF interference and, conversely, prevent RF from escaping from the module and causing interference with external apparatus. The housing carrier or wall also serves as a thermal transmission path, allowing heat from the circuit boards, internally generated inside the module housing, to pass to a heat sink or otherwise dissipate in the environment.
The module houses a variety of populated printed circuit boards carrying the electronic circuits that together defines the purpose of the particular module in the electronic system. Those circuit boards, populated with electronic chips and other electronic components, are placed in positions about the available space in the rectangular area on the carrier wall inside the module housing that are assigned by the circuit designer. Since the electronic functions of the circuit boards typically differ from one another, individual circuit boards may vary in size and height, but all fit within the available space on the module housing carrier and within the internal volume of the module housing.
With the top wall or cover of the module housing removed, the inside region appears compartmentalized, a second physical characteristic of the internal region. The inside region is divided by a variety of metal walls upstanding from the bottom wall and the individual circuit boards fit inside respective regions defined by two or more associated metal walls. The upstanding walls may appear in a variety of lengths and some contain bends, producing open and/or closed compartments in a variety of sizes. The metal walls are attached to and extend from the bottom carrier wall up to the plane of the inner surface of the top cover so that, with the top wall, the module cover, fastened in place, the upstanding walls, cover and bottom carrier define a number of metal three-dimensional cavities or regions, each of which contains a circuit board that is populated with electronic components or microstrip lines.
The upstanding metal walls of the module serve as RF shielding to prevent RF energy as may be generated in and radiated from a given circuit board from propagating to another circuit board by an undesired route that could cause improper functioning of the electronic circuits or render the electronic circuits dysfunctional. The complexity and variety of two-dimensional shapes defined by the internal walls inside a typical module is visible in the isometric view of a typical module presented in FIG. 1, which may be inspected briefly.
As those skilled in the art recognize, individual circuit boards are typically fastened to the bottom wall of the module by solder, which holds the respective boards in place and places the electrical ground of the circuit board in common with the metal walls of the module housing. The bottom side of each printed circuit board, which may be a laminate, contains a metal outer surface, typically a solder coating. That metal outer surface is pre-tinned as required by the conventional solder reflow process. By pressing the circuit board against the metal bottom wall of the module and re-flowing the solder, that is, heating the solder (and other components in the vicinity) to the eutectic temperature of the solder, after which the heat is removed, the solder resolidifies and bonds the printed wiring board in place. This last briefly summarized step is one that produces the greatest expense and effort in the assembly process, which the present invention addresses and improves upon.
In existing practice pressing a circuit board against the bottom wall of the module during the solder reflow process requires use of a metal block, often formed of titanium or aluminum. The circuit board is clamped between the metal block and the metal wall and then heat is applied. With many circuit boards in a module, there must be many blocks, all of which are clamped for the solder reflow process.
The metal block must fit in the space occupied by the associated circuit board, and between any of the upstanding metal walls adjoining the respective circuit board. The metal block must also be of sufficient height to reach the top of the module and be accessible to the clamp. As earlier explained (and as is apparent from FIG. 1) each of the compartments of the module""s real estate may be of a different size and shape. Hence, the associated metal block for a respective region must be individually machined to shape. A typical module may require thirty or more individually machined metal clamping blocks. Thus, once the layout of the module is finalized by the module designer and ready for manufacture, the manufacturing engineers must prescribe the size of the blocks needed for the manufacturing process.
The blocks need to be machined to shape, typically to tolerances of thousandths of an inch. As those skilled in the art appreciate the fabrication of the metal blocks is a time consuming procedure and makes manufacturing set-up expensive. Moreover, since volume manufacturing of communications equipment using such modules may be limited, the cost of the foregoing set up on a per unit basis is quite high. Since an electronic system contains a number of different modules of different function, and, hence, possess different layouts, a great many different metal blocks must be machined to shape, perhaps hundreds of such blocks. And pity the engineer who determines that it is necessary to change the layout once the electronic module design has been released for manufacture.
A principal reason for using a clamping procedure during the solder reflow process is to ensure that the entire bottom surface of each circuit board is soldered to the metal wall, particularly around the edges of the circuit board. As example, should the side edge of a circuit board be uplifted from the board due to ineffective soldering, leaving a gap, the gap increases the length of the ground path to the next circuit board in the electronic circuit in the module. If the RF output from the circuit board is propagating a high frequency microwave signal to an adjacent microstrip line or another circuit board, as example, the ground path for return current is increased, undesirably introducing attenuation of the signal. Moreover, due to the metal sides of the wall and circuit board, the gap forms a microwave cavity that may be resonant at the frequency of RF being propagated from the circuit board, which may result in oscillations or signal path power losses.
Use of metal blocks is also slightly disadvantageous to the solder reflow process. Since the metal block is heat transmissive, the block conducts heat away from the solder, requiring more heat be applied than otherwise.
As an advantage, the present invention obsoletes metal clamping blocks and renders those blocks unnecessary for the solder reflow process in module fabrication. The printed circuit boards may be solder bonded to the bottom wall of the module by reflowing the solder by pressing the printed circuit board against that wall and reflowing the solder, but without use of metal clamping blocks.
Accordingly, an object of the invention is to enhance the efficiency of the process of manufacturing electronic modules and reduce manufacturing cost.
A further object of the invention is to reduce the time and cost of set-up required for the manufacture of electronic modules, and, more specifically for reflow soldering of circuit boards in place in the module housing.
And a still further object of the invention is to press printed circuit boards against the metal surface during reflow soldering of those circuit boards to the metal surface without using metal clamping blocks.
In accordance with the invention, solder-backed circuit boards are placed in assigned positions on the carrier metal and then the inside volume of the electronic module housing is filled to the top with particulate, such as small beads, while leaving the edges of any upstanding metal shields in the inside volume visible. A plate backed sheet of foam material, e.g. a foam sheet, is placed over the module housing and is then clamped down, pressing against the particulate, and, through the particulate, pressing the confined circuit boards against the carrier metal. The clamped assembly is then placed in a heating chamber or other reflow processing station and is heated to reflow the solder on the back of the circuit boards. The heating is withdrawn and the assembly is allowed to cool down, completing the reflow soldering. The particulate is removed from the housing interior (and may be reused), completing the soldering process.
In accordance with a more specific aspect of the invention the particles of the particulate are glass spheres. And, in accordance with a further specific aspect of the invention, the particulate is of material that is no better than poor in thermal conductivity.
The foregoing method allows soldering of a variety of circuit boards of a variety of size and shape to assigned locations on the base or carrier metal of the electronic module to be completed all at once. By eliminating custom made metal blocks previously used to clamp the circuit boards against the carrier metal the complex and expensive work to fabricate those metal blocks to shape in setting up for solder reflow processing is avoided; and the assembly of circuit boards to the electronic module housing is greatly simplified.